Recent progress in interfacial photo-vapor conversion technology using metal sulfide-based semiconductor materials

Highlights

• Metal sulfide-based semiconductors (MSSCs) as photothermal materials were discussed.

• Preparation and application of MSSCs in photothermal conversion were discussed.

• Factors affecting distillation efficiencies and performance of MSSCs were discussed.

• Challenges and criticism of MSSMs based solar evaporation systems were suggested.

Abstract

Interfacial photo-vapor conversion technology (IPVCT), as a promising technology to solve the shortage of drinking water, has attracted increasing attention. Metal sulfide-based semiconductor materials (MSSMs) possessing unique internal structures, high optical absorption, water delivery channels, and desired thermal transportation capability. MSSMs have showed an excellent photo-vapor conversion performance. The current review summarizes the basic mechanisms of IPVCT based on various MSSMs. We firstly discuss the current development in enhancing photothermal conversion efficiency such as the thermal management and optimization of the internal water channels of photothermal materials. Importantly, we present the current state of art in the applications of IPVCT based on MSSMs, as well as wide of other implementation beyond a steam generation. Finally, we highlight the research gaps in the field of MSSMs in IPVCT, and suggest the future research directions. This is to evoke joint fundamental and industrial research efforts toward cost-effective, practical and useful solar evaporation systems based on MSSMs.

Introduction

The shortage of drinking water has become a global problem with the growth of the world population and the rapid industrious development [1]. The gap between demand and supply of water is narrowing down. It is necessary to develop effective technology for obtaining safe drinkable water from seawater and industrial wastewater that account for a large proportion of water resources [2]. Presently, the effective technologies used for treating high salinity water include membrane technology [3], reverse osmosis [4] and electrolysis [5]. However, these technologies suffered by high cost and complicated processes, so it is extremely urgent to develop a cost-effective treatment technology for potable water [6], [7]. For the past decades, researchers have focused on a solar-driven water evaporation technology that can effectively purify high salinity water into clean water [8], [9].

Solar energy is regarded as a green, clean and unlimitedly renewable source. It can be widely used in photocatalysis [10], [11], power generation [12] and photo-distillation [13]. In classical photo-distillation technologies, the solar irradiation is firstly absorbed and converted into heat energy through a solar-thermal material, and then the water is heated to form steam followed by condensation for further collecting clean water [14], [15], [16]. However, the classical photo-distillation technologies encounter several obstacles, such as high heat energy loss and low optical absorption rate. To surmount above limitations, the concept of interfacial photo-vapor conversion was proposed, in which the photothermal conversion efficiency can be highly enhanced by optimizing the structure of the evaporator [17], [18], [19]. The interfacial photo-vapor conversion mainly occurs at the air-water interface, in which surface water is locally heated by solar absorbers. By the means of local heating, the heat loss can be greatly reduced, accelerating the conversion rate of water transformed from liquid phase to gas phase. In this process, the photothermal conversion efficiency mainly relies on the performance of the photothermal material, as it is one of the most important parts of photo distillation equipment. For purpose to improve the photothermal conversion efficiency, various solar absorbers have been highly developed, such as carbon based materials [20], [21], [22], plasma materials [23], [24], and semiconductor materials [25], [26], [27]. Carbon nanotubes (CNTs) and graphene (GR), as the most concerned carbon-based photothermal materials so far, achieved nearly 100% optical absorption in the whole solar spectra range, but the preparation process of CNTs and GR is very complicated and consumes a large amount of heat energy [28]. In addition, the hydrophobicity of CNTs and GR inhibits the continuous transport of pure water for evaporation [29], [30]. Plasma materials exhibit excellent photothermal conversion performance, but most of them are made of precious metals, which would be highly cost in large-scale applications [31]. In contrast, semiconductor materials are considered as excellent photothermal conversion materials that have been potentially used in practical applications due to their stable photothermal conversion efficiency, good chemical stability and abundant natural resources [32]. It is also possible to adjust the band gap of the semiconductor materials to tune their spectral absorption ranging in the UV–Vis–NIR bands. This is the most significant advantage of semiconductor materials to be applied in IPVCT [33].

For the past decades, many efforts have been made to explore semiconductor materials (e.g., metal sulfides and metal oxides) as photothermal materials and the mechanism of photothermal conversion has been further understood. Comparably, it was found that metal oxides have a wide band gap, for example, the band gap of WO₃ is 3.4 eV [34]. Generally, a narrow band gap of metal oxides can be obtained through complicated adjustment of phase transition or doping with other elements, while pure metal sulfides has a narrow band gap (~2 eV) [35]. Therefore, the metal sulfide based-semiconductor materials (MSSMs) supposed in IPVCT has received a great deal of attention. In one hand, this kind of materials is ubiquitous in nature and relatively inexpensive to produce [36]. In the other hand, compared to others semiconductor materials, MSSMs have high biocompatibility, rich category and low cytotoxicity. A simple preparation process can obtain most of them and they exhibited porous structure with high specific surface area [37]. Moreover, some MSSMs almost processes 100% optical absorption capacity in the whole solar spectrum [38]. Due to the plentiful vertical water delivery channels and efficient heat transfer capability, MSSMs showed amazing photothermal conversion efficiency [39]. It is also possible to further optimize the water delivery channels and the hydrophobic characteristics of the surface to achieve a higher steam generation efficiency [40]. Several kinds of photothermal materials explored in the IPVCT have been recently reviewed, among most of them are carbon-based materials [14], [16], [41]. Therefore, we summarize the development of MSSMs in photothermal conversion to illustrate the research progress and the basic mechanism of these novel materials.

Herein, the state of the art and basic mechanisms of IPVCT are firstly described. Then, MSSMs with different internal structures and properties are particularly illustrated. Furthermore, current development in enhancing photothermal conversion efficiency such as the thermal management and optimization of the internal water channels of photothermal materials are discussed. In this review, we focus on the recent development of MSSMs in IPVCT in recent years, but also to present some suggestions and prospects in this field.